A blast furnace-converter process has been known as an iron-making process using iron ore as a raw material. The blast furnace converter process is a process of producing steel by reducing iron ore in a blast furnace to produce molten iron containing carbon in high concentration, and decarburizing the molten iron in a converter. The above blast furnace-converter process requires preliminary treatment of raw materials such as coke and sintered ore. Further, in recent years, it has a tendency to become large in scale in order to enjoy a scale merit, which reduces flexibility or productivity to resources. Further, from the viewpoint of natural environmental protection, an iron-making process for reducing CO2 gas emissions is desired. However, the above blast furnace-converter process is a so-called indirect iron-making process, so that it has a problem that the CO2 gas emissions are large compared to a direct iron-making process in which steel is directly produced by reducing iron ore. For this reason, in recent years, the direct iron-making process is recognized once again.
As the above direct iron-making process, for example, a MIDREX process has been known. In the MIDREX process, a large amount of natural gas is used as a reducing agent for reducing iron ore. For this reason, there has been a drawback that a location place of a plant is limited to a production area of natural gas.
Therefore, a process using easily available coal as a reducing agent instead of the natural gas has recently attracted attention. In this process, granular metallic iron is produced by charging an agglomerate including an iron oxide-containing material such as iron ore and a carbonaceous reducing agent such as coal onto a hearth of a heating furnace such as a movable hearth furnace, reducing iron oxide in the agglomerate by heating due to gas heat transfer or radiant heat from heating burners in the furnace, and melting reduced iron by further heating to cause the reduced iron to coalesce. This process has advantages that high-speed reduction becomes possible because powdery iron ore can be used as it is and the iron core and the reducing agent are closely arranged, and that the carbon content in a product can be adjusted by a method such as adjustment of the blending amount of the reducing agent.
In producing granular metallic iron in a movable hearth type thermal reduction furnace, the present inventors disclose technology of Patent Document 1 as a method which can produce high-quality granular metallic iron having a high C amount and a low S amount. In this technology, the flow rate of atmospheric gas in the furnace is controlled, in producing the granular metallic iron by charging an raw material mixture including an iron oxide-containing material and a carbonaceous reducing agent onto a hearth of a movable hearth type thermal reduction furnace and heating it, reducing iron oxide in the raw material mixture with the carbonaceous reducing agent, melting metallic iron produced, causing the metallic iron melted to coalesce into granules while separating it from slag formed as a by-product, and then, cooling and solidifying it. Specifically, the average flow rate of the atmospheric gas in the furnace is controlled to 5 m/sec or less, and this flow rate control is performed at least between an end stage of reduction and completion of melting of the metallic iron.